This invention relates generally to improvements in magnetic bearing structures. More particularly, the present invention relates to a magnetic bearing structure which utilizes the combination of a controllable radially polarized electromagnetic field and a relatively constant axially polarized magnetic field, to suspend a rotatable member relative to a stationary member in a stable manner.
Electromagnetic bearings are highly effective for supporting a body, such as a rotating shaft, which is effectively floated or levitated by magnetic fields. In this way the rotating shaft has no frictional contact with any stationary structure, thereby permitting relatively friction free rotation of the shaft or rotation of a body about the shaft. This arrangement possesses the obvious advantage that there is no mechanical abrasion, which results in reduced mechanical noise and durability not available with other types of bearing structures. Moreover, because of the reduced frictional effects which would otherwise be encountered with conventional bearing structures, it is possible to obtain higher speeds of rotation with electromagnetic bearings.
Magnetic bearings typically require little maintenance and readily lend themselves to operation in hostile environments such as in connection with corrosive fluids where other conventional bearings would be destroyed or rendered inoperable. Further, magnetic bearings are suitable for supporting moving bodies in a vacuum, such as in outer space, or in canned pumps where the pump rotor must be supported without the use of physically contacting bearings.
Conventional electromagnets utilized for energizing levitation gaps are inefficient in that they require a substantial amount of electrical power to generate the required electromagnetic field. In general, prior electromagnetic bearings require large electromagnetic coils and electronic-controlled circuitry which have been found to be inherently inefficient. There have been some proposals to use permanent magnets in combination with electromagnets in order to provide greater stabilization and control. However, the conventional prior designs, which utilize both electromagnets and permanent magnets, are inefficient from a spacial standpoint and are considerably complex.
One of the primary considerations in the development of magnetic bearing structures is to eliminate so-called air gaps. The so-called air gaps form a portion of the magnetic flux pathway of the electromagnets and permanent magnets, and provide a bridge between a supporting structure and a levitated structure. In actuality, some air gaps must be tolerated in order to position a suspended or rotatable body. Thus, air gaps to some extent cannot be avoided, but it is desirable to reduce air gaps to an absolute minimum.
From a pure physics standpoint, an air gap introduces great inefficiency into any type of magnetic structure. An air gap is about 2,000 times less efficient than an iron core medium for transmitting magnetic flux. Thus, in terms of inefficiency, a magnetic bearing structure which has an air gap of 0.1 inch is far more inefficient than a magnetic bearing which has an iron gap of 20 inches.
In addition, it is important to overcome the conductivity constraints of permanent magnets. Essentially, permanent magnets are very poor conductors for a magnetic flux, even though they generate magnetic flux. The most efficient permanent magnets available are the rare earth alloy magnets. Such permanent magnets, however, have a very low magnetic permeability and they behave in much the same manner as air gaps in the magnetic circuit. The low permeability of rare earth alloy magnets require significant power to drive electromagnetic fields through the permanent magnets, thereby resulting in low electrical efficiencies. Thus, it is undesirable to transmit an electromagnetic field through a permanent magnet.
Accordingly, there has been a need for a novel electromagnetic bearing structure which utilizes both a radially polarized, controllable electromagnetic field and an axially polarized constant magnetic field to produce a compact and spacially efficient structure which is lightweight and obtains a high power efficiency. Additionally, there exists a need for an electromagnetic bearing structure wherein magnetic efficiency of the device is optimized by minimizing air gaps between the levitated and support structures, and wherein the electromagnetic coils are not required to provide magnetomotive forces to drive magnetic flux through permanent magnets. Further, such an electromagnetic bearing structure is needed in which relatively small electromagnetic coils may be employed to maintain the levitated structure in a desired position through use of a servo control circuit. Such a novel bearing structure should lend itself to concurrent use of electromagnets and permanent magnets for the purpose of providing a high density, constant magnetic flux between the associated structures, and should permit configuration of the magnetic bearing structure to rotate a levitated shaft within a housing, as well as a levitated cylinder generally encircling the support structure. The present invention fulfills these needs and provides other related advantages.